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Shaft-driven bicycle
A shaft-driven bicycle is a chainless bicycle that uses a drive shaft instead of a chain to transmit power from the pedals to the wheel. Shaft drives were introduced over a century ago, but were mostly supplanted by chain-driven bicycles due to the gear ranges possible with sprockets and derailleurs. Recently, due to advancements in internal gear technology, a small number of modern shaft-driven bicycles have been introduced. Shaft-driven bikes have a large bevel gear where a conventional bike would have its chain ring. This meshes with another bevel gear mounted on the drive shaft. The use of bevel gears allows the axis of the drive torque from the pedals to be turned through 90 degrees. The drive shaft then has another bevel gear near the rear wheel hub which meshes with a bevel gear on the hub where the rear sprocket would be on a conventional bike, and canceling out the first drive torque change of axis. The 90-degree change of the drive plane that occurs at the bottom bracket and again at the rear hub requires the use of bevel gears. Bevel gears are the most efficient way of turning drives 90 degrees as compared to worm gears or crossed helical gears. The drive shaft is often mated to a hub gear which is an internal gear system housed inside the rear hub. Today, there are three significant manufacturers of internal hubs suitable for use with shaft drive systems, including Shimano Nexus, SRAM and Sturmey-Archer. Comparison of shaft vs chain on the rear wheel of a shaft-drive bicycle]] A valid comparison of shaft vs. chain drives can only be made if both bikes use the same type of gearing, whether single-speed or with an internal gear system. Most of the advantages claimed for a shaft drive can be realized by using a fully-enclosed chain case. Advantages *'Compactness:' The drive shaft and bevel gears take up less volume than an enclosed chain. *'Ground clearance:' The drive shaft system allows greater ground clearance. *'Consistency of performance:' Shaft drives operate at a very consistent rate of efficiency and performance, without adjustments or maintenance. *'Gear shifting:' Since shaft-drives require gear hubs for shifting at all, they gain the benefit that gears can be shifted while the bicycle is at a complete stop or moving in reverse. *'Enclosed drive system:' Enclosing the drive shaft and gears provides several benefits: :*'Safety:' There is no danger of clothing or fingers being drawn into the enclosed gears. :*'Cleanliness:' Enclosed gears cannot get grease on hands or clothing. :*'Longevity:' Enclosed gears keep their performance stable for a long time. :*'Reduced maintenance and repair:' Enclosed gears are less susceptible to damage and require less adjustment or cleaning. Disadvantages * Power loss: A shaft-driven system is less efficient than a chain-driven system. Shaft driven systems are often quoted as being "95% efficient", while chain driven systems are quoted as "98% efficient". * Complexity: Though the shaft-driven system seems simple on the outside, the manufacture of the necessary bevel gears is complex compared to that of a chain and sprockets. * Cost: Due to the manufacturing complexity of the frame and shaft drive, the manufacturing cost is typically higher. * Gear range: Internal hub geared bikes typically have a more restricted gear range than comparable derailleur-equipped bikes, and have fewer ratios within that range (an issue because a rider typically prefers to pedal within a fairly narrow cadence band). * Weight: A shaft-driven bicycle typically weighs about one pound (454 grams) more than an equivalent chain-driven bicycle. * Changing a flat: shaft drives are typically more complex to disassemble when repairing punctures. Some of the other issues addressed by the shaft drive, such as protection for clothing and from ingress of dirt, can be met through the use of chain guards. The reduced need for adjustment in shaft-drive bikes also applies to a similar extent to chain or belt-driven hub-geared bikes. Not all hub gear systems are shaft compatible. Bicycles typically employ more torque (an adult man can easily generate more than 100 ft-lb) but at much lower rotational speed than motorcycles and cars. This mitigates against shaft drive as the shafts must be large to cope with the torque, which adds to their mass. This is particularly noticeable when pedaling hard, when there can be noticeable torsional springing in the drive train. Past and present The shaft-driven bicycle was originally developed in the late 1800s. The French company L'Acatane did most of the development in the 1890s. During this period, Columbia aggressively marketed the chainless bicycle in the USA. Chainless bicycles were moderately popular in 1898 and 1899, although sales were still much smaller than regular bicycles, primarily due to the high cost. The bikes were also somewhat less efficient than regular bicycles — there was roughly an 8 percent loss in the gearing, in part due to limited manufacturing technology at the time. The rear wheel was also more difficult to remove to change flats. Many of these deficiencies have been overcome in the past century. In 1902, The Hill-Climber Bicycle Mfg. Company sold a three-speed shaft-driven bicycle in which the shifting was implemented with three sets of bevel gears. While a small number of chainless bicycles were available, for the most part, shaft-driven bicycles disappeared from view for most of the 20th century. There is, however, still a niche market for chainless bikes, especially for commuters, and there are a number of manufacturers who offer them either as part of a larger range or as a primary specialization. See also * Velovision magazine regularly reviews shaft-drive bikes. References External links *Schematic of drive system *Detailed discussion on gearing *Drive Shaft Bicycle Web Page *Giovelli's "Shaft-Drive" system Category:Cycle types ja:シャフトドライブ mk:Кардански велосипед nl:Fiets met cardanas